1. Field of the Invention
The present invention relates to a method of manufacturing a heating element of the molybdenum silicide type and also to a heating element.
2. Description of the Related Art
An electric resistance element of the molybdenum silicide type is described in Swedish Patent Specifications 0003512-1 and 0004329-9. According to patent specification 0003512-1 the resistance material of the heating element includes Mo(Si1-xAlx)2 which contains aluminum to an extent at which the formation of pest is essentially prevented.
It has been found that when such material is operated in a temperature range of 400–600° C. no pest, or only a slight amount of pest, is formed. Pest is formed by virtue of the formation of MoO3 from MoSi2 and O2.
The reason why the formation of pest is significantly reduced or is eliminated is due the formation of Al2O3 on the surface of the element.
According to one preferred embodiment x is caused to lie in the range of 0.2–0.6.
The other patent specification, 0004329-9, teaches a method of increasing the useful life span of heating elements that consist chiefly of molybdenum silicide and alloys of that basic material where the element operates at high temperatures.
According to that patent specification, the heating element contains aluminum to an extent which is sufficient to maintain a stable, slowly growing layer of aluminum oxide on the surface of the heating element.
According to a preferred embodiment the heating element material contains Mo(Si1-xAlx)2, where x lies in the range of 0.2–0.6.
A material of the molybdenum silicide type that contains aluminum has been found to possess improved corrosion properties at both low and high temperatures.
Such material is often produced by mixing MoSi2 powder with oxidic raw material, such as aluminosilicates. When the raw material is bentonite clay, there is obtained a relatively low melting point which contributes towards so-called smelt phase sintering, which results in dense materials that contain MoSi2 and a proportion of aluminum silicate corresponding to 15–20 percent by volume.
Bentonite clay has different compositions. Some bentonites include 60% by weight SiO2 while some contain somewhat more than 70% by weight SiO2. Although the Al2O3 content varies, it normally lies between 13–20% by weight. The melting point varies between about 1200–1400° C.
Bentonite clay that contains chiefly SiO2 can be used in the production of heating elements containing Mo(Si1-xAlx)2. When sintering with an Al-alloyed silicide there takes place a chemical exchange reaction in which the greater affinity of the oxygen to Al than to Si results in Si leaving the aluminum silicate and entering the silicide as a result of Al leaving the silicide and being taken up by the oxide phase. That exchange reaction also contributes towards improved sintering properties of the composite material. The final material contains Mo(Si1-xAlx)2 that is substantially depleted of Al, where the oxide phase contains Al2O3 in all essentials.
The standard procedure of manufacture involves mixing molybdenum, silicon, and aluminum in powder form and firing the powder mix normally under a shielding gas atmosphere. This results in a cake of the material Mo(Si1-yAly)2, where y is larger than x as a result of said exchange reaction. The reaction is exothermic. The cake is then crushed and ground down to a fine particle size normally in the order of 1–20 μm. The resulting powder is mixed with bentonite clay to form a wet ceramic material. The material is extruded and dried to a rod form whose diameter corresponds to the diameter of the subsequent heating element. The material is then sintered at a temperature that exceeds the melting temperature of the included components.
However, there is a drawback with an element of that kind. The problem is that the oxide that forms on the surface of the element, namely Al2O3, sometimes peels away or flakes off, i.e., loosens from the surface of the element, in cyclic operation.
A peeling oxide gives poorer protection against continued oxidation of aluminum, which becomes impoverished in the outer surface of the element more quickly. Moreover, peeling oxide can contaminate the oven in which the element is fitted, with the risk that performance and the appearance of products heat treated in ovens that have such elements will be significantly impaired. This restricts the use of such elements in heating processes.
This problem is solved by the present invention.